Optical fibers commonly consist of a glass portion (typically with a diameter of about 120-130 μm), inside which the transmitted optical signal is confined, The glass portion is typically protected by an outer coating, typically of polymeric material. This protective coating typically comprises a first coating layer positioned directly onto the glass surface, also known as the “primary coating”, and of at least a second coating layer, also known as “secondary coating”, disposed to surround said first coating. In the art, the combination of primary coating and secondary coating is sometimes also identified as “primary coating system”, as both these layer are generally applied during the drawing manufacturing process of the fiber, in contrast with “secondary coating layers” which may be applied subsequently. In this case, the coating in contact with the glass portion of the fiber is called “inner primary coating” while the coating on the outer surface of the fiber is called “outer primary coating”. In the present description and claims, the two coating layers will be identified as primary and secondary coating, respectively, and the combination of the two as “coating system”.
The thickness of the primary coating typically ranges from about 25 μm to about 35 μm, while the thickness of the secondary coating typically ranges from about 10 μm to about 30 μm.
These polymer coatings may be obtained from compositions comprising oligomers and monomers that are generally crosslinked by means of UV irradiation in the presence of a suitable photo-initiator. The two coatings described above differ, inter alia, in the mechanical properties of the respective materials. As a matter of fact whereas the material which forms the primary coating is a relatively soft material, with a relatively low modulus of elasticity at room temperature, the material which forms the secondary coating is relatively harder, having higher modulus of elasticity values at room temperature. The coating system is selected to provide environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending, which can lead to attenuation of the signal transmission capability of the fiber and is therefore undesirable. In addition, coating system is designed to provide the desired resistance to physical handling forces, such as those encountered when the fiber is submitted to cabling operations.
The optical fiber thus composed usually has a total diameter of about 250 μm. However, for particular applications, this total diameter may also be smaller; in this case, a coating of reduced thickness is generally applied.
In addition, as the operator must be able to identify different fibers with certainty when a plurality of fibers are contained in the same housing, it is convenient to color the various fibers with different identifying colors. Typically, an optical fiber is color-identified by surrounding the secondary coating with a third colored polymer layer, commonly known as “ink”, having a thickness typically of between about 2 μm and about 10 μm, or alternatively by introducing a colored pigment directly into the composition of the secondary coating.
Among the parameters which characterize primary and secondary coatings performances, elastic modulus and glass transition temperature of the cross-linked materials are those which are generally used to define the mechanical properties of the coating. When referring to the elastic modulus it should be clarified that in the patent literature this is sometimes referred to as “shear” modulus G (or modulus measured in shear), while in some other cases as “tensile” modulus E′ (or modulus measured in tension). The determination of said elastic moduli can be made by means of DMA (Dynamic mechanical analysis) which is a thermal analysis technique that measures the properties of the materials as they are deformed under periodical stress. For polymeric materials, the ratio between the two moduli is generally 1:3, i.e. the tensile modulus of a polymeric material is typically about three times the shear modulus (see for instance the reference book Mechanical Properties and Testing of Polymers, pp. 183-186; Ed. G. M. Swallowe)
Examples of coating systems are disclosed, for instance, in U.S. Pat. No. 4,962,992. In said patent, it is stated that a soft primary coating is more likely to resist to lateral loading and thus to microbending. It thus teaches that an equilibrium shear modulus of about 70-200 psi (0.48-1.38 MPa) is acceptable, while it is preferred that such modulus being of 70-150 psi (0.48-1.03 MPa). These values correspond to a tensile modulus E′ of 1.4-4.13 MPa and 1.4-3.1 MPa, respectively. As disclosed in said patent, a too low equilibrium modulus may cause fiber buckling inside the primary coating and delamination of the coating system. In addition, said patents suggests that the glass transition temperature (Tg) of the primary coating material should not exceed −40° C., said Tg being defined as the temperature, determined by means of stress/strain measurement, at which the modulus of the material changes from a relatively high value occurring in the lower temperature, glassy state of the material to a lower value occurring in the transition region to the higher temperature, elastomeric (or rubbery) state of the material.
Other examples of coating compositions are disclosed, for instance, in WO 01/05724, which discloses radiation curable fiber optic coating materials comprising a (meth)acrylate urethane compound derived from a polypropylene glycol or comprising a (meth)acrylate urethane compound derived from a polypropylene glycol and a further polyol including a polyester polyol. These compositions may be used, once cured, as coating material for optical fibers and optical fiber ribbons, including primary coatings, secondary coatings, coloured secondary coatings, inks, matrix materials and bundling materials. In the introductory part, said document mentions that primary coatings should in particular have a very low Tg.
However, as noticed by the Applicant, although a primary coating has a relatively low value of Tg (as generally required by the art), the value of the modulus of the coating material may nevertheless begin to increase at temperatures much higher than the Tg, typically already above 0° C. Thus, while a low value of Tg simply implies that the transition of said coating from its rubbery to its glassy state takes place at relatively low temperatures, no information can be derived as to which would be the variation of the modulus upon temperature decrease. As a matter of fact, an excessive increase of the modulus of the primary coating upon temperature decrease may negatively affect the optical performances of the optical fiber, in particular at the low temperature values, thus causing undesirable attenuation of the transmitted signal due to microbending.
Thus, as observed by the applicant, what seems important for controlling the microbending of an optical fiber is the temperature at which the coating material begins the transition from its rubbery state (soft) to its glassy state (hard), which temperature will be referred in the following of this specification and claims as the “hardening temperature” of the material, or Th. In particular, attention should be paid to select a composition which still shows a relatively low modulus at said Th, so that an excessive increase of the modulus upon further temperature decrease can be avoided.
In the present description and claims, the term “modulus” is referred to the modulus of a polymeric material as determined by means of a DMA test in tension, as illustrated in detail in the test method section of the experimental part of the present specification.
In the present description and claims, the term “hardening temperature” is referred to the transition temperature at which the material shows an appreciable increase of its modulus (upon temperature decrease), thus indicating the beginning of an appreciable change from a relatively soft and flexible material (rubber-like material) into a relatively hard and brittle material (glass-like material). The mathematical determination of Th will be explained in detail in the following of the description.
According to the present invention, the Applicant has thus found that attenuation losses caused by microbending onto a coated optical fibers, particularly at the low exercise temperatures, can be reduced by suitably controlling the increase of the modulus at the low temperatures. In particular, the Applicant has found that said microbending losses can be reduced by using a polymeric material for the primary coating having a low hardening temperature and a comparatively low modulus at said temperature. In addition, the Applicant has found that by selecting coating compositions having a relatively low equilibrium modulus, said attenuation losses can be further controlled over the whole operating temperature range.